A Stirling cycle engine depends very importantly on the operation of a thermal regenerator disposed between the expansion and compression spaces of the closed working fluid system. Although regeneration has been studied for quite a period of time in connection with the operation of the Stirling engine, its true theoretical basis of operation is not completely understood. However, the regenerator is designed with certain practical operating conditions in mind. The design of such regenerator assumes that the temperatures of the working fluid at the inlet to the regenerator matrix will be at a certain minimum temperature level, such as 80.degree. C. The design further assumes that even though the inlet temperature to the matrix will cyclically vary because the compression-expansion of the heat input is other than isothermal, the assumption is that such variation will be relatively, small within the range of .+-.30.degree. C. Similarly, the temperature at the exit of the matrix, varying as a practical matter because of inlet variance and because limited coefficients of heat transfer, it is assumed will not vary considerably and will be within the limits of, for example, 750 .+-. 50.degree. C. With these temperature conditions in mind, the designer then selects a certain desirable heat capacity for the regenerator at a certain void volume so as to provide a compromise between tolerable fluid friction therethrough, loss in pressure and optimum heat transfer characteristics.
The resultant regenerator, as designed with these considerations in mind by the prior art, does not compensate for the cold working condition from which a Stirling engine must be started. If a significant goal of the Stirling engine is to be realized, which includes dramatic fuel savings over that of prior art engines, the fuel consummed in raising the temperature of the working fluid from a cold starting condition must be reduced.
Adding additional heat to the expansion space to decrease the amount of time that it takes to raise the working fluid medium to a proper operating temperature is not an adequate solution by itself. This is in part due to the fact that the blow time which is defined to be the net time for flow through the dead space of the system between expansion and compression spaces, including the void volume within the regenerator, is extremely short when compared to other prior art engines, such as a gas turbine engine. For example, at moderate engine speeds of 1200 revolutions per minute, the blow time is 10 times less than that of the permissable mimimum in the gas turbine. In fact, in an engine, which is of moderate size adaptable for vehicular use as a prime mover, the blow time will be so short that many particles of working fluid will never pass completely through the matrix of the regenerator before the flow direction is reversed. The very short net flow time through the matrix in one direction is slightly less than half the complete cycle time. Accordingly, the conventional heat transfer process which occurs through the regenerator is very complex and incomplete, involving repetitive fluid-to-matrix, matrix-to-fluid, fluid-to-matrix cycle relationships.
What is needed is a mechanism or method by which the working fluid of a Stirling cycle engine can be moved rapidly from a cold starting state to an operating temperature condition without reliance upon the normal external circuit or the normal transfer of heat from the external heating circuit through the conventional compression-expansion cycle. If the latter were to be the only alternative solution, it would be hindered by fluid friction within the working cycle and the need for a larger void volume within the regenerator to speed up the temperature increase of the matrix. All of this would work at odds with the desire for efficient operation at high temperature conditions.